Field of the Invention
The present disclosure relates to a tire-reinforcing steel cord for a vehicle, and more particularly to a tire-reinforcing steel cord having a double layer structure in which a first-layer core has an elliptical or rectangular cross section, and a radial tire using the same. The tire-reinforcing steel cord can improve processability, fatigue characteristics, and rolling resistance of a tire, leading to an increase in fuel efficiency of a vehicle.
Description of the Related Art
A steel cord is a construction of several steel wires and is obtained by plating carbon steel wires with brass, subjecting the brass-plated carbon steel wires to a drawing process, and twisting the drawn wires in the combination of 1×3, 1×4, 2+2, 2+7, 3+6, 3+9+15, or the like according to use of the manufactured steel wire. The steel cord is superior to any other reinforcing material in terms of strength, modulus, thermal resistance, fatigue resistance, etc., so that it is being used as material for reinforcing rubber articles such as tires, conveyer belts, etc.
Generally a steel belt used for a tire is obtained by arranging 300 to 600 steel cords, in each of which multiple steel wires are twisted, side by side, and placing and rolling topping rubber layers on the top and bottom surfaces of the arranged steel cords. It is necessary for the steel cords to have strong adhesion to the rubber layers and have excellent durability in a harsh environment while a vehicle is running.
In the rolling-processed structure which has undergone rolling, tensile strength occurs in a compressed direction due to an elastic recovery characteristic of the steel cord. When the rolling-processed structure is cut into pieces, bending deformation of the rubber layers and the steel cords is likely to occur, and thus there is difficulty in automatic connection work in a subsequent process. Accordingly, it is important to control the bending deformation. In order to improve workability in a subsequent tire manufacturing process, when the rolling-processed structure is cut into pieces each having a predetermined unit length, structural safety is required. That is, it is necessary to prevent ends of a piece of the rolling-processed structure having the unit length obtained through the cutting from floating or sinking. Adjustment of rolling of the steel cords is important in maintaining structural stability of rubber-topped rolling-processed steel cords.
The rolling of the steel cord is referred to as residual rotation. When a torsion torque is applied to each steel wire filament to deform the steel wire such that the steel wire is twisted in a predetermined manner, the filament which is elastically deformed tends to be untwisted in a direction opposite to a twisting direction due to the elastic recovery characteristic, and a filament which has undergone excessive plastic deformation tends to be further twisted in the same direction as the twisting direction. Accordingly, after the steel cord is manufactured, residual rotation occurs.
In the steel cord industry, an overtwister, prestress roller, etc. are used to stabilize residual rotation or prevent occurrence of residual rotation. However, since it is difficult to fundamentally prevent the rolling attributable to a torsion deformation force applied to filaments when manufacturing the steel cord by twisting multiple steel filaments in a direction, improvement in this aspect is required.
Generally, a steel cord for a heavy duty tire is prepared by twisting multiple filaments of two layers or three layers. This steel cord has a dense structure in which there is no gap between the steel filaments. Rubber infiltration into the inside of the steel cord is poor. Therefore, if a tire is repeatedly bent by external force, damages of both mechanical wear and chemical corrosion, i.e., fretting fatigue occurs due to friction between the filaments and infiltration of moisture or salt, respectively. This deteriorates durability of a tire.